1. Field of the Invention
The present invention relates to a thermal stencil plate making method. More particularly, the invention relates to a thermal stencil plate making method using a thermal stencil original sheet in which a thermoplastic resin film and a porous support are laminated to each other. Further, the invention relates to a thermal stencil plate making method using a thermal stencil original sheet substantially made only of a thermoplastic resin film.
2. Description of the Related Art
The screen printing method is conventionally widely used as a simple and easy printing method. This method employs a thermal stencil original sheet in which a thermoplastic resin film is layered on the surface of an appropriate ink-transmissive support. The thermoplastic resin film is heated to fuse by a thermal head or the like to form image-like perforations. Printing ink is introduced from the ink-transmissive support side to effect printing on a printing object such as paper. Alternatively, using a thermal stencil original sheet substantially only of a thermoplastic resin film, the thermoplastic resin film is heated to fuse by a thermal head to form image-like perforations and printing ink is forced through the single thermoplastic resin film to effect printing on a printing object such as paper (in the case of support-less stencil).
The thermal heads for thermal color development recording or for thermal transfer recording have been divertedly used heretofore as the thermal head for the above thermal stencil plate making method. The thermal heads for thermal recording are designed to form a record image of continuous pixels connected to each other. If such thermal heads were used for thermal stencil plate making, the image of perforations would be continuous, which results in increasing offset or in increasing plate wear. Meantime, the diameter of image perforations can be changed by adjusting the energy applied to the thermal head. The perforation diameter becomes larger as the applied energy increases; conversely, it becomes smaller as the applied energy decreases. Employing such technique, there is a method to assure the independency of image perforations. However, the most suitable application energy is to be in the energy range in which the dispersion of perforations in a perforation image is smallest. This condition is generally also to select the maximum application energy in the range in which the durability of the thermal head is increased. Decreasing the applied energy below the suitable value increases the dispersion of image perforations, while increasing the applied energy is not preferable in respect of the durability of the thermal head. Further, a new problem of support-less thermal stencil original sheet appeared such as an abnormal image (especially wrinkles) arisen from plate shrinkage due to thermal shrinkage of a non-image portion.
To solve the above problems in the thermal stencil original sheet with support, there is means of reducing only the thickness of the protection layer of heating elements, proposed in Japanese Patent Application Laying Open (KOKAI) No. 63-191654. However, since the thickness of the protection layer is made extremely thin as 0.5 to 3.5 .mu.m, there are a lot of pin holes in the protection layer. When the humidity is high, an antistatic agent enters the pin holes to cause corrosion of the electrode. Even if the humidity is not so high, the pin holes could cause lack of abrasion resistance. In addition, in case that the thickness of the protection layer is 0.5 .mu.m, the image perforations are liable to be connected to each other if a distance between heating elements is below 2.5 .mu.m (d/T&lt;5). Further in this case, especially with the support-less thermal stencil original sheet, the new problem of abnormal image for example of wrinkles is liable to arise from plate shrinkage. Conversely, if the thickness of protection layer is 3.5 .mu.m and if a distance between heating elements is longer than 35 .mu.m (10&lt;d/T), the image perforations are separate too far from each other. Especially in case of support-less thermal stencil original sheet, a printed image will lose its continuity, because an ink transfer amount is small because of the property thereof.
Proposed in Japanese Patent Application Laying Open (KOKAI) No. 2-67133 is means to make the secondary scan length of each heating element in the thermal head shorter than the dot pitch in the primary scan direction. It was, however, insufficient, because the master, to which the invention was directed, was the thermal stencil original sheet with support. Wrinkles due to plate shrinkage sometimes appeared especially in making a stencil plate from a support-less stencil original sheet. The degree of wrinkle appearance increases as the film thickness of support-less stencil original sheet becomes thicker. Further, even with a general thermal stencil original sheet, the communication between perforations occurred if the dot pitch in the primary scan direction is made different from that in the secondary scan direction. Especially with support-less thermal stencil original sheet, too far image perforations were sometimes made for the same reason as described above.